NIOS Lesson 26 - ALCOHOLS, PHENOLS AND ETHERS
Hydrocarbon derivatives are compounds formed when one or more hydrogen atoms in a hydrocarbon are replaced by other atoms or functional groups, giving rise to substances like alcohols, aldehydes, ketones, amines, esters, ethers, and carboxylic acids. These derivatives are crucial in fuels, pharmaceuticals, plastics, and everyday chemicals.
๐ฌ What Are Hydrocarbon Derivatives?
- Definition: Hydrocarbons are molecules made only of carbon and hydrogen. When a hydrogen atom is substituted with another atom or group (like –OH, –NH₂, –COOH), the result is a hydrocarbon derivative.
- Formation: Example: Methane (CH₄) becomes methanol (CH₃OH) when one hydrogen is replaced by a hydroxyl group.
- Importance: They expand the chemical diversity of hydrocarbons, enabling applications in fuels, solvents, medicines, and polymers.
⚗️ Major Classes of Hydrocarbon Derivatives
| Functional Group | Example Compound | Key Uses |
|---|---|---|
| Alcohols | Ethanol, Methanol | Fuels, solvents, beverages |
| Aldehydes | Formaldehyde | Preservatives, plastics |
| Ketones | Acetone | Solvents, nail polish remover |
| Amines | Aniline | Dyes, pharmaceuticals |
| Esters | Ethyl acetate | Perfumes, flavorings |
| Ethers | Diethyl ether | Anesthetics, solvents |
| Carboxylic acids | Acetic acid | Vinegar, industrial chemicals |
๐ Applications in Daily Life
- Fuels: Ethanol and biodiesel are renewable energy sources.
- Pharmaceuticals: Many drugs (like aspirin) are hydrocarbon derivatives.
- Plastics: Polymers such as polyethylene terephthalate (PET) are derived from hydrocarbons.
- Perfumes & Flavors: Esters provide fruity aromas and tastes.
⚠️ Risks & Considerations
- Toxicity: Some derivatives (like methanol, formaldehyde) are highly toxic.
- Environmental Impact: Hydrocarbon derivatives can contribute to pollution if not managed properly.
- Sustainability: Bio-based derivatives (like bioethanol) are being developed to reduce reliance on fossil fuels.
So far you have learnt the chemistry of hydrocarbons which serve as basic skeleton for the attachment of various functional groups to give a large number of their derivatives.
In the last lesson, we discussed one such class of compounds viz halogen derivatives of hydrocarbons. Another very useful and important catagory of hydrocarbon derivatives is that of compounds containing functional groups in which the carbon atom is linked to an oxygen atom.
We have devoted two lessons for the study of these compounds. In this lesson, you will study about compounds containing carbon-oxygen single bond (–C–O) whereas the next lesson deals with compounds containing carbonoxygendouble-bond ( C O).
Among the compounds with carbon-oxygen single bond are the classes of alcohols, phenols and ethers having the following general structures.
These are very important categories of compounds both in the industry and in the synthesis of other organic compounds. You will study each of these classes of compounds in this Lesson.
OBJECTIVES
๔บ classify alcohols as primary, secondary or tertiary;
๔บ name simple alcohols according to IUPAC system of nomenclature;
๔บ list general methods of preparation of alcohols;
๔บ discuss the properties of alcohols in the light of their structure;
๔บ explain various reactions exhibited by alcohols to give other categories of organic compounds;
๔บ describe important uses of alcohols;
๔บ give the names of common phenolic compounds;
๔บ describe the laboratory and industrial methods of preparation of phenols;
๔บ explain the greater acidity of phenols as compared to alcohols;
๔บ discuss the reactions of phenols;
๔บ name ethers according to the IUPAC system of nomenclature;
๔บ describe the general methods of preparation of ethers and
๔บ explain the important reactions of ethers.
26.1 ALCOHOLS
Alcohols are organic compounds that have one or more hydroxy (-OH) groups
bonded to the carbon atoms in aliphatic compounds. They occur widely in nature and have many industrial and pharmaceutical applications. For example, methanol and ethanol are two industrially important alcohols.
26.1.1 Classification and Nomenclature of Alcohols
Alcohols are classified as primary (1ยบ), secondary (2ยบ) or tertiary (3ยบ) depending upon whether the number of alkyl groups bonded to the carbon atom bearing the hydroxy group is one, two or three, respectively.
According to the IUPAC system of nomenclature, alcohols are called alkanols.
They are named as the derivatives of the corresponding alkane in which the -e of the alkane is replaced by -ol .
The procedure for nomenclature involves the following steps:
Step 1: Select the longest carbon chain which contains the carbon atom bearing the –OH group. Count the number of carbon atoms and identify the corresponding alkane. From the name of this alkane, drop the final e and suffix -ol in its place.
This gives the root name or the parent name.
Step 2: Number the carbon chain starting from the end nearest to the hydroxy group. The number of the carbon atom bearing the hydroxy group is indicated before -ol in the name.
Step 3: Number the other substituents according to their position on the chain.
Step 4: Write the name of the alcohol by listing the substituents in the alphabetical order alongwith their position.
You may remember from Lesson 25 that the hydroxyl group takes precedence over double and triple bonds.
Table 26.1 illustrates some common alcohols and their IUPAC and common names. Go through them in light of the steps given above for nomenclature.
In the above examples, only one –OH group is present in the molecule. These Compoundsalcohols are called monohydric alcohols. Alcohols having two hydroxyl groups in a molecule are known as dihydric alcohols or diols or glycols. Examples of some diols are shown below :
Note that the term glycol generally means 1,2-diol or a vicinal diol. In these diols, the two hydroxyl groups are present on the adjacent carbon atoms.
Similarly, alcohols having three hydroxyl groups are called trihydric alcohols. 1,2,3-propanetriol which is commonly known as glycerol, is a trihydric alcohol.
26.1.2 General Methods of Preparation
Alcohols are synthesized by the following general methods. You might have come across some of these methods in previous lessons. Let us now study these methods.
1. Hydrolysis of Haloalkanes
Haloalkanes can be converted to corresponding alcohols using aqueous sodium or potassium hydroxide or water as nucleophiles.
2. From hydration of Alkenes
Hydration means addition of water molecule. In case of alkenes, hydration is the addition of H+ and OH– across the double bond to give alcohols.
Alkenes can be hydrated by the following methods:
(i) Acid-catalysed Hydration
Alkenes can be hydrated to yield alcohols in the presence of acid catalysts.
The reaction proceeds via alkyl hydrogen sulphate and this method is used for
the industrial preparation of ethanol.
In case of unsymmetric alkenes, the addition follows Markovniokov’s rule.
This method gives very good yield of alcohols and here also, the addition takes Compounds
place in Markovnikov’s fashion.
Lithium aluminium hydride reduces all of the above classes of compounds while sodium borohydride reduces only aldehydes and ketones and does not reduce carboxylic acids and esters. Hence, it can be used to selectively reduce aldehydic/ketonic carbonyl group in presence of carboxylic acid/ester function. Compounds Some examples below illustrate the use of these reagents.
5. Diazotization of Primary Aliphatic Amines
This reaction also yields alchols and will be discussed in Lesson 30.
6. Fermentation
Ethanol is prepared on a large scale using fermentation. It involves breaking down large molecules into simpler ones using enzymes. Usually, yeast is added as a source of enzymes.
The fermentation of sugar is shown below :
You know that the electronegativity of oxygen is more than that of hydrogen.
Therefore, in alcohols, the O–H bond is polar in nature. In other words, oxygen has a slight negative charge on it whereas hydrogen has a slight positive charge.
This bond polarity alone cannot explain the higher boiling points of alcohols as compared to hydrocarbons or similar haloalkanes, as listed in Table 26.2.
Normally, hydrogen bonding is responsible for higher boiling points of alcohols.
Hydrogen bonding amongst alcohol molecules is depicted in Fig. 26.2.
You can see that the negatively polarised oxygen atom of one alcohol molecule Compounds
attracts the positively polarised hydrogen atom of the other molecule. Thus, alcohol molecules are associated or are held together. This force of attraction is to be overcome before a molecule is set free from the liquid state and vaporises. Thus, more heat energy is required to break the hydrogen bonds and hence, the boiling points of alcohols are higher than alkanes and haloalkanes of comparable molecular mass.
Table 26.2: Physical Properties of some Alcohols, Hydrocarbons and related Haloalkanes
From the last column of Table 26.2, you must have noticed that alcohols have
high solubilities in water. The lower alcohols are completely miscible and their
solubilities decrease as the hydrocarbon portion of the molecule becomes larger.
The higher solubility of alcohols can be again attributed to the hydrogen bonding.
In this case, hydrogen bonding takes place between the alcohol and water
molecules as is shown below in Fig. 26.3.
primary alcohols < secondary alcohols < tertiary alcohols
With primary alcohols turbidity does not appear. In case of secondary alcohols, turbidity appears within 5 mintues whereas it appears immediately with tertiary alcohols. The turbidity is due to the formation of alkyl chlorides from the corresponding alcohols.
4. Formation of Alkenes
Alcohols can be dehydrated to alkenes. This reaction requires an acidic catalyst and is favoured at higher tempratures. Usually sulphuric and phosphoric acid are used as acidic catalysts. You have come across this reaction in Lesson 26 also. The ease of dehydration follows the following order amongst alcohols.
tertiary alcohols > secondary alcohols > primary alcohols
26.1.4 Mechanism of Dehydrations of Alcohols
The dehydration of alcohols to give alkenes follows an E1 mechanism. The reaction is acid catalysed. Here, E stands for dimination and 1 denotes that it is unimolecular in nature which means only one molecule is involved in the rate determining step.
The reaction occurs in three steps: Compounds
(i) Protonation of the oxygen of OH group
(ii) Loss of water to give a carbocation
(iii) Loss of a proton from a neighboring carbon atom which produces an alkene
This is shown below:
The formation of ethers by dehydration is a substitution type of reaction and gives only symmetrical ethers. You will study a better method of synthesis of ethers later under the section of ethers in this lesson.
6. Oxidation
Alcohols can be oxidised to carbonyl compounds. Primary alcohols give aldehydes or carboxylic acids on oxidation while secondary alcohols yield
The aldehydes obtained by oxidation of the primary alcohols get further oxidized to carboxylic acids as shown above. You will study more about these classes of compounds in the next lesson.
The oxidation can be controlled, and aldehydes are obtained as the products by using pyridium chlorochromate (PCC) which is a mild reagent.
This reaction is called esterification reaction and is reversible in nature.
Uses
Alcohols find a large variety of uses as follows :
1. As solvents
2. As laboratory reagents
3. In medicines
4. As thinners in paints, varnishes, etc.
Uses of Methanol and Ethanol
Methanol is toxic to humans and even its small quantities can cause blindness.
In larger quantities, its consumptions can cause death. However, it is useful industrially both as a solvent and in the production of formaldehyde and acetic acid, as a starting material. It is also used in the synthesis of which is used as an to gosoline MTBE
Ethanol, obtained by the fermentation of sugars is present in all alcoholic beverages.
It is also an important industrial chemical. Thus, ethanol used for scientific and Compounds industrial purposes is made purposely toxic to prohibit people from consuming it.
Ethanol can be used as a fuel and as a fuel additive. It is also used as a solvent in the manufacture of varnishes and perfumes.
It is also used in hand sanitizers as an antiseptic.
INTEXT QUESTIONS 26.1
1. Give the IUPAC names of the following alcohols:
(i) 2-Methylpentan-2-ol
(ii) 2-Ethylbut-2-en-1-ol
(iii) 1, 4-Pentanediol
2. How will you prepare propan-1-ol from propanal?
3. Give the synthesis of 2-methylpropan-2-ol using Grignard reagent.
4. Give the product of the following reactions:
5. What is MTBE? Give its use.
MTBE is methyl tert-butyl ether. It is used as an additive to gasoline.
6. What is the product obtained when alcohols are dehydrated? Alkenes
26.2 PHENOLS
The name phenol is specifically used for the following compound (hydroxybenzene) in which one hydroxyl group is attached to the benzene ring.
It is also used as a general name for the class of compounds derived from the above compound. Phenol is a disinfectant. Phenols are widely distributed in nature. They are also important in the synthesis of organic compounds such as aspirin and in the preparation of dyes. Phenol is also used in the manufacture of bakelite which is a very useful polymer.
26.2.1 Nomenclature of Phenols
Some representative examples of phenolic compounds are given below:
Note that the term phenol is used as a parent name and the other substituents present in the compound are given a specific number according to their position on the aromatic ring. As done before the common names of the above compounds are given in the brackets below their IUPAC names.
26.2.2 General Methods of Preparation
We can categorize the methods of preparation as methods of laboratory synthesis and industrial synthesis of phenols.
A. Laboratory Synthesis of Phenols
1. From Arenediazonium Salts
It is the most general method of preparation of phenols and requires mild conditions.
Arenediazonium salts or aromatic diazonium salts are obtained by the diazotization Compounds of primary aromatic amines as given below :
2. Alkali Fusion of Sodium Benzenesulphonate
This was the first commercial synthesis of phenol developed in Germany in 1890.
It can also be used as a laboratory method for synthesis of phenol.
Sodium benzenesulphonate is fused with sodium hydroxide to give sodium phenoxide which on acidification yields phenol.
B. Industrial Synthesis of Phenols
1. Dow Process
In this process, chlorobenzene is heated with aqueous sodium hydroxide under pressure. Sodium phenoxide so produced on acidification gives phenol.
This method was in use for many years but now phenol is synthesised via cumene
hydroperoxide which is discussed below.
2. From Cumene Hydroperoxide
The reaction between benzene and propene in presence of phosphoric acid yields
cumene.
26.2.3 Physical Properties Compounds
Similar to alcohols, phenols also have hydrogen atom linked to the electronegative oxygen atom. Thus, phenols also exhibit hydrogen bonding and hence have higher boiling points as compared to the hydrocarbons of similar molecular weight.
Due to their ability to form hydrogen bonds, phenols show some water solubility.
For example, the solubility of phenol is 9.3 g per 100 mLof water.
26.2.4 Reactions of Phenols
Let us now study the reactions exhibited by phenols.
1. Acidic and Basic Nature
Phenols are much more acidic than alcohols. pKa values of some phenols are
listed in Table 26.4.
The greater acidity of phenols can be attributed to the resonance stabilization of the phenoxide ion. The resonance structures of phenoxide ion are shown in Fig. 26.4.
The delocalisation of the negative charge over the benzene ring stabilises the phenoxide ion. No such stabilisation is possible, in case of alkoxide ions.
Similar resonance is also shown in phenol itself, see Fig 26.5. But the resonance structures of phenol are less stable as compared to those of phenoxide ion as they involve the separation of charge.
If you carefully go through the pKa values given in Table 26.4, you would see that the electron donating substituents such as methyl group decrease the acidity of phenol and hence alkylphenols have greater pKa values as compared to phenol itself. On the other hand, electron withdrawing substituents increase the acidity and phenols having these substituents (–Cl, –NO2, etc.) have lower pKa values than phenol. In fact, 2,4,6-trinitrophenol is more acidic than many carboxylic acids.
Phenols behave as weak bases also. Similar to alcohols, they can also be protonated to give phenyloxonium ion.
2. Electrophilic Substitution Reactions Compounds
The hydroxyl group is a powerful activating group and hence phenols readily undergo electrophilic substitution reactions. In this reaction, an electrophile (electron loving species) attacks the benzene ring and replaces one of its hydrogen atoms. Since the ortho and para positions of the phenol are electron rich, the substitution takes place at these positions. Two such reactions are halogenation and nitration reactions. Let us now study them in details.
(i) Halogenation: Phenol reacts with bromine in aqueous solution to give 2,4,6-tribromophenol in about 100% yield.
Bromination can be limited to monobromination to give mainly 4-bromophenol using low temprature and less polar solvent such as carbon disulphide. The other product formed in minor quantity is 2-bromophenol.
(ii) Nitration: Phenol gives a mixture of 2-nitro and 4-nitrophenols on nitration with dilute nitric acid.
The mixture of nitrophenols so obtained is separated using steam distillation.
Both these products show hydrogen bonding. In case of 2-nitrophenol, the hydrogen bonding is intramolecular (in the same molecule) whereas in case of 4-nitrophenol, it is intermolecular (between different molecules). These are depicted in Fig. 26.5.
2-Nitrophenol is steam volatile and distills out on passing steam whereas
4-nitrophenol is less volatile due to intermolecular hydrogen bonding.
Treatment of phenol with a mixture of conc. nitric acid and conc. sulphuric acid at 323K yields 2,4,6-trinitrophenol also known as picric acid.
3. Kolbe Reaction
It involves sodium phenoxide which is allowed to absorb carbon dioxide and then heated under a pressure of CO2 to 398 K. Sodium salicylate so obtained on acidification yields salicylic acid.
By reaction with acetic anhydride, salicylic acid yields aspirin, which is the common pain reliever.
4. Oxidation
Phenols undergo oxidation reactions to give products which are diffrent from those obtained by alcohols. They can be oxidised using a variety of oxidising agents such as sodium dichromate or silver oxide to give quinones. These days Fremy’s salt [(KSO3)2NO] is preferred for oxidation.
5. Reimer Tiemann Reaction Compounds
Phenols react with chloroform in the presence of sodium hydroxide (or potassium hydroxide) solution followed by acidification to give hydroxy aldehydes.
Uses
1. Phenol is used as a disinfectant.
2. It is also used in the synthesis of polymers.
3. Phenols are used in the synthesis of many organic compounds.
4. Substituted phenols are used in dyeing and tanning industries.
INTEXT QUESTIONS 26.2
1. How will you convert aniline to phenol?
2. What is the starting material in Dow’s process? Chlorobenzene
3. Arrange the following in the increasing order of their acidity:
Phenol, 2-Methylphenol, 2-Chlorophenol
2-Methylphenol < Phenol < 2-Chlorophenol
4. How will you prepare salicylic acid from phenol? By Kolbe reaction
5. What is an azo dye?
Azo dyes are azo compounds formed by the reaction of phenols with aromatic diazonium salts. They are brightly coloured.
26.3 ETHERS
Ethers are organic compounds in which an oxygen atom is bonded to two alkyl groups or aryl groups. Thus, ethers can be represented as R −O − R′ where R and R′ may be alkyl or aryl groups. When the two substituent groups (R and R′ ) are identical, then the ether is called a symmetrical ether, otherwise if these two groups are different, then the ether is known as an unsymmetrical ether.
Ethers are commonly used as solvents for organic reactions. The symmetrical ether shown above is diethyl ether and is commonly also referred to simply as ether because of its wide use as a solvent for reactions and extraction of organic compounds. It was also used as an anaesthetic for over hundred years.
26.3.1 Nomenclature of Ethers Compounds
Common names of ethers have arrived by alphabetically naming the two groups attached to the oxygen followed by the word ether. The common names for some ethers are given below:
In IUPAC nomenclature, the larger alkyl (or aryl) group is used as the root name as the alkane and the smaller alkyl group is treated as an alkoxy substituent on this alkane. For example, in ethyl methyl ether having ethyl and methyl groups, the ethyl group is larger than methyl group and hence this ether is treated as the ethane derivative.
26.3.2 General Methods of Preparation
You have already studied under the reactions of alcohols that ethers can be obtained by the dehydration of alcohols. Ethers can also be prepared by Williamson synthesis which is explained below:
Williamson Synthesis: It involves the reaction of a metal alkoxide with a primary alkyl halide. The metal alkoxide is prepared by adding sodium or potassium metal or sodium hydride (NaH) to the alcohol.
Williamson synthesis involves the displacement of the halide ion by the alkoxide ion.
26.3.3 Structure and Properties of Ethers
Ethers have geometry similar to water and alcohols. The oxygen atom is sp3 hybridised. There are two lone pairs of electrons present on the oxygen atom as is shown in Fig. 26.6.
Note that the ether molecule has a bent structure. Since the carbon-oxygen bond is polar and the molecule has a bent structure, there is a net dipole moment and the ether molecule is polar is nature (Fig. 26.7). Ethers, thus, act as polar solvents.
Since ethers do not have a hydrogen atom linked to the oxygen atom, they cannot
form hydrogen bonds amongst their own molecules. Due to the absence of hydrogen bonding, they have lower boiling points as compared to alcohols having similar molecular masses. The boiling points of some ethers are listed in Table 26.5.
26.3.4 Reactions of Ethers
Ethers are normally unreactive in nature. Their unreactivity makes them good solvents. However, they show some reactions which are discussed below :
1. Reaction with Oxygen : Ethers slowly react with oxygen to form hydroperoxides and peroxides.
Peroxides have a tendency to explode. Therefore, one should be very careful in handling ethers which may have been stored for sometime because they may contain some peroxide.
2. Reaction wtih Acids
Since the oxygen atom of ethers contains lone pairs of electrons, they can accept a proton from the acids. Thus, ethers are basic in nature.
INTEXT QUESTIONS 26.3
1. What are the IUPAC names of the following ethers ?
(i) 2-Methoxybutane
(ii) Methoxymethane
2. (i) How will you prepare methyl propyl ether using Williamson synthesis?
(ii) What is the IUPAC name of methyl propyl ether ?
Methoxypropane
3. Why should you be careful in using old stock of ethers.
They may explode due to the presence of peroxides.
4. Why are ethers good solvents?
Because they are unreactive in nature.
5. What is tetrahydrofuran? Give its structure and use.
It is a cyclic ether.
It is used as a solvent.
WHAT HAVE YOU LEARNT?
In this lesson, you have learnt that
๔บ Alcohols can be classified as primary, secondary or tertiary.
๔บ Alcohols can be monohydric, dihydric or polyhydric.
๔บ Alcohols can be prepared by the following general methods:
– Hydrolysis of haloalkanes
– Hydration of alkenes
– Reduction of carbonyl compounds
– From aldehydes and ketones using Grignard reagents
๔บ Alcohols behave both as weak acids and weak bases.
๔บ Alcohols can be converted to alkyl halides, alkenes, ethers, aldehydes, ketones, carboxylic acids and esters.
๔บ In the laboratory, phenols can be prepared from arenediazonium salts and sodium benzene sulphonate. Their industrial preparation is done by Dow’s process and from cumene hydroperoxide.
๔บ Similar to alcohols, phenols can also behave both as acids and bases.
๔บ Typical reactions of phenols being electrophilic substitution reactions such as halogenation, sulphonation, nitration, etc.
๔บ Phenols undergo oxidation and also exhibit Reimer Tiemann reaction. They react with aromatic diazonium salts to give azo dyes.
๔บ Ethers can be symmetrical or unsymmetrical.
๔บ Ethers can be prepared by Williamson synthesis.
๔บ Dialkyl ethers are cleaved on heating with strong acids.
TERMINAL EXERCISE
1. Give the IUPAC names of the following compounds:
2. Compare the boiling points of ethyl alcohol and dimethyl ether. Which one has higher boiling point and why?
3. Which ester would give ethanol and methanol on reduction?
4. Complete the following reactions:
5. How is ethanol prepared using fermentation?
6. What is Lucas test? What is its use?
7. Which reagent is used for oxidising primary alcohols to aldehydes?
8. Why are phenols more acidic than alcohols? Explain.
9. Why are ethers polar in nature?
Alcohols, phenols, and ethers are three major classes of organic compounds distinguished by the presence of the hydroxyl (–OH) group in alcohols and phenols, and the alkoxy (–OR) group in ethers. They are widely used in pharmaceuticals, antiseptics, solvents, fuels, and fragrances, making them central to both industry and daily life.
๐ฌ Structure and Classification
Alcohols
- Contain one or more hydroxyl (–OH) groups attached to a carbon atom.
- Classified by number of –OH groups:
- Monohydric: Ethanol (C₂H₅OH)
- Dihydric: Ethylene glycol (used in antifreeze)
- Trihydric: Glycerol (used in medicines, cosmetics)
- Further classified as primary (1°), secondary (2°), or tertiary (3°) depending on the carbon atom bonded to the –OH group. flexbooks.ck12.org
Phenols
- Hydroxyl group directly attached to an aromatic ring (benzene).
- Example: Phenol (C₆H₅OH), used in antiseptics like Dettol.
- Stronger acids than alcohols due to resonance stabilization of phenoxide ion. NCERT
Ethers
- General formula R–O–R′, where R and R′ are alkyl or aryl groups.
- Classified as:
- Symmetrical: Diethyl ether (C₂H₅–O–C₂H₅)
- Unsymmetrical: Methyl ethyl ether (CH₃–O–C₂H₅)
- Historically used as anesthetics; now solvents in labs. Learn CBSE
⚗️ Preparation Methods
Alcohols:
- Hydration of alkenes (Markovnikov’s rule).
- Reduction of aldehydes/ketones.
- Grignard reagent reactions.
Phenols:
- Hydrolysis of diazonium salts.
- From cumene (industrial process).
Ethers:
- Williamson synthesis (reaction of alkyl halides with sodium alkoxides).
๐งช Chemical Properties
Alcohols:
- Reactions involving O–H bond cleavage (formation of alkoxides).
- Oxidation: Primary → Aldehyde → Carboxylic acid; Secondary → Ketone.
Phenols:
- Undergo electrophilic substitution (ortho/para directing).
- Kolbe’s reaction → Salicylic acid.
- Reimer–Tiemann reaction → Salicylaldehyde.
Ethers:
- Cleavage of C–O bond with strong acids.
- Aromatic ethers undergo electrophilic substitution.
๐ Comparison Table
| Compound | Functional Group | Example | Key Uses |
|---|---|---|---|
| Alcohols | –OH on aliphatic carbon | Ethanol | Solvent, fuel, beverages |
| Phenols | –OH on aromatic ring | Phenol | Antiseptics, resins |
| Ethers | R–O–R′ | Diethyl ether | Solvent, anesthetic |
๐ Everyday Relevance
- Alcohols: Ethanol in sanitizers and spirits, glycerol in cosmetics.
- Phenols: Chloroxylenol in Dettol, salicylic acid in aspirin.
- Ethers: Diethyl ether in early anesthesia, now used in labs.
Perfect — let’s dive deeper into alcohols, phenols, and ethers with a focus on their preparation methods, chemical reactions, and industrial applications.
-
⚗️ Preparation Methods
Alcohols
- Hydration of alkenes (acid-catalyzed addition of water).
- Reduction of aldehydes, ketones, and carboxylic acids.
- Fermentation of sugars (ethanol production).
- Grignard reagent reaction with carbonyl compounds.
Phenols
- Hydrolysis of diazonium salts.
- From cumene (industrial process for phenol production).
- Chlorobenzene hydrolysis under high pressure and temperature.
Ethers
- Williamson ether synthesis (alkyl halide + sodium alkoxide).
- Acid-catalyzed dehydration of alcohols.
๐งช Chemical Reactions
Alcohols
- Oxidation: Primary → Aldehyde → Carboxylic acid; Secondary → Ketone.
- Esterification with carboxylic acids.
- Dehydration to form alkenes.
Phenols
- Electrophilic substitution (ortho/para directing).
- Kolbe’s reaction → Salicylic acid.
- Reimer–Tiemann reaction → Salicylaldehyde.
Ethers
- Cleavage with strong acids (HI, HBr).
- Aromatic ethers undergo electrophilic substitution.
๐ญ Industrial Applications
Alcohols
- Ethanol: fuel, solvent, beverages, sanitizers.
- Methanol: antifreeze, fuel, formaldehyde production.
- Glycerol: cosmetics, pharmaceuticals, food industry.
Phenols
- Antiseptics (Dettol contains chloroxylenol).
- Plastics and resins (Bakelite).
- Precursors for drugs like aspirin.
Ethers
- Diethyl ether: solvent, historically used as anesthetic.
- MTBE (methyl tert-butyl ether): fuel additive.
- Used in perfumes and pharmaceuticals.
๐ Quick Comparison
Compound Preparation Key Reaction Industrial Use Alcohols Hydration of alkenes Oxidation, esterification Fuels, solvents, sanitizers Phenols Cumene process Electrophilic substitution Antiseptics, plastics, drugs Ethers Williamson synthesis Acid cleavage Solvents, fuel additives
Would you like me to illustrate these with reaction mechanisms (step-by-step diagrams of how bonds break and form), or focus more on their real-world applications in medicine and industry?
1. Alcohols: Dehydration to Form Alkenes ($C-O$ and $O-H$ Cleavage)
Step 1: Protonation ($O-H$ Bond Formation)
- Oxygen uses a lone pair to attack an acid hydronium ion ($H_3O^+$).
- Bond Formed: $O-H$ bond.
- Result: An alkyloxonium ion ($\text{R}-\text{OH}_2^+$). [11, 12, 13]
Step 2: Leaving Group Departure ($C-O$ Bond Breaking)
- The highly polar $C-O$ bond breaks heterolytically.
- Both electrons in the bond move completely to the oxygen atom.
- Bond Broken: $C-O$ bond.
- Result: A stable water molecule ($H_2O$) leaves, leaving behind a reactive carbocation intermediate ($\text{R}_2\text{CH}-\text{CH}_2^+$).
Step 3: Deprotonation ($C-H$ Bond Breaking, $C=C$ Bond Formation)
- A weak base (like water) attacks the hydrogen on the adjacent ($\beta$) carbon.
- The $C-H$ bonding electrons collapse down to form a new carbon-carbon pi ($\pi$) bond.
- Bond Broken: $\beta$ $C-H$ bond.
- Bond Formed: $C=C$ double bond.
- Result: An alkene product. [21, 22, 23, 24]
2. Phenols: Williamson Ether Synthesis ($O-H$ Cleavage)
Step 1: Deprotonation ($O-H$ Bond Breaking) [30]
- A strong base (such as $NaOH$) abstracts the acidic proton from the hydroxyl group.
- The two electrons from the $O-H$ bond collapse entirely onto the oxygen.
- Bond Broken: $O-H$ bond.
- Result: A resonance-stabilized phenoxide ion ($\text{Ar-O}^-$). [31, 32]
Step 2: Nucleophilic Substitution / $S_N2$ ($C-O$ Bond Formation)
- The negatively charged phenoxide oxygen acts as a nucleophile. It attacks an unhindered alkyl halide ($\text{R}-\text{X}$).
- Simultaneously, the carbon-halogen ($C-X$) bond breaks as the halide leaving group departs.
- Bond Formed: A new ether $C-O$ bond.
- Bond Broken: $C-X$ electrophile bond.
- Result: An alkyl aryl ether.
3. Ethers: Acidic Cleavage by Hydrogen Halides ($C-O$ Cleavage)
Step 1: Protonation of Ether Oxygen ($O-H$ Bond Formation) [44, 45]
- The ether oxygen uses one of its lone pairs to grab a proton ($H^+$) from the strong acid ($HX$).
- Bond Formed: $O-H$ bond.
- Result: A highly reactive protonated ether (dialkyloxonium ion). This turns one of the alkyl chains into a viable leaving group. [46, 47, 48, 49]
Step 2: Nucleophilic Attack via $S_N2$ ($C-X$ Formed, $C-O$ Broken)
- The halide ion ($\text{X}^-$) generated in Step 1 acts as a nucleophile. It attacks the less sterically hindered carbon atom adjacent to the oxonium oxygen.
- As the new $C-X$ bond forms, the shared electrons in the ether $C-O$ bond shift completely over to the oxygen atom.
- Bond Formed: $C-X$ bond.
- Bond Broken: Ether $C-O$ bond.
- Result: An alkyl halide ($\text{R-X}$) and an alcohol molecule ($\text{R'-OH}$).
Summary Table of Bond Changes
| Functional Group [50, 51, 52, 53, 54] | Key Reaction Type | Bonds Broken | Bonds Formed | Intermediate Stage |
|---|---|---|---|---|
| Alcohol | Acid-Catalyzed Dehydration | $C-O$, $\beta$ $C-H$ | $O-H$, $C=C$ | Oxonium Ion $\rightarrow$ Carbocation |
| Phenol | Williamson Ether Synthesis | $O-H$, $C-X$ | $C-O$ | Phenoxide Anion |
| Ether | Acidic Cleavage ($HX$) | $C-O$, $H-X$ | $O-H$, $C-X$ | Dialkyloxonium Ion |
In chemistry, “cleavage of elements” usually refers to the breaking of chemical bonds within molecules, known as bond cleavage. This process can occur in two main ways: homolytic cleavage (equal sharing of electrons, forming radicals) and heterolytic cleavage (unequal sharing, forming ions).
๐ฌ Types of Bond Cleavage
Homolytic cleavage
- Each atom takes one electron from the bond.
- Produces free radicals (species with unpaired electrons).
- Represented with fishhook arrows (half-headed).
- Example: Cl₂ → Cl• + Cl• (chlorine radicals).
Heterolytic cleavage
- One atom takes both bonding electrons.
- Produces ions (cation + anion).
- Represented with full-headed arrows.
- Example: C–Br → C⁺ + Br⁻ (carbocation + bromide ion).
⚗️ Energy Considerations
- Bond dissociation energy: Energy required to break a bond homolytically.
- Homolytic cleavage is usually endothermic (requires energy input).
- Heterolytic cleavage depends on electronegativity and solvent effects (polar solvents stabilize ions).
๐ Comparison Table
| Type of Cleavage | Electron Distribution | Products | Example |
|---|---|---|---|
| Homolytic | Equal (1 electron each) | Radicals | Cl₂ → Cl• + Cl• |
| Heterolytic | Unequal (both to one atom) | Ions | C–Br → C⁺ + Br⁻ |
๐ Applications
- Homolytic cleavage: Basis of free radical reactions (polymerization, combustion).
- Heterolytic cleavage: Key in ionic reactions (SN1, SN2, electrophilic substitution).
- Used in organic synthesis, drug design, and industrial chemistry.
Esterification is a fundamental organic reaction in which a carboxylic acid reacts with an alcohol to form an ester and water. It’s one of the most important reactions in organic chemistry because esters are widely used in fragrances, flavors, solvents, and plastics.
⚗️ General Reaction
[ RCOOH + R'OH \xrightarrow{H^+} RCOOR' + H_2O ]
- Reactants: Carboxylic acid (R–COOH) + Alcohol (R′–OH)
- Catalyst: Concentrated H₂SO₄ (acts as dehydrating agent and proton donor)
- Products: Ester (R–COOR′) + Water
๐ฌ Step-by-Step Mechanism
Protonation
- Acid catalyst protonates the carbonyl oxygen of the carboxylic acid, increasing electrophilicity.
Nucleophilic attack
- Alcohol oxygen attacks the carbonyl carbon, forming a tetrahedral intermediate.
Proton transfer
- Rearrangement stabilizes the intermediate.
Elimination
- Water molecule leaves, forming the ester.
Deprotonation
- Final ester product is neutralized.
๐ Example
- Acetic acid + Ethanol → Ethyl acetate + Water
[ CH_3COOH + C_2H_5OH \xrightarrow{H^+} CH_3COOC_2H_5 + H_2O ]
Ethyl acetate has a fruity smell and is used in perfumes and nail polish removers.
๐ Applications
- Perfumes: Esters give pleasant fruity and floral aromas.
- Food flavoring: Banana flavor (isoamyl acetate), pineapple flavor (ethyl butyrate).
- Solvents: Ethyl acetate in paints and varnishes.
- Plastics: Polyesters used in fabrics and bottles.
Comments
Post a Comment